Protection Method Of Low-resistance Grounding System Fault

The 10 kV low-resistance grounding system is suitable for the situation where there are more cables in the substation due to the increasing power demand in modern times.It can quickly remove the faulty line and reduce the impact of faulty capacitor current and overvoltage on the system.The widely-used relay protection of the low-resistance grounding system is essentially definite time zero-sequence overcurrent protection.The configuration is simple and practical,but in the case of high-transition resistance grounding or multi-line multi-phase grounding,it may lose sensitivity or selectivity.Therefore,it is necessary and important to solve the relay protection problems of high transition resistance grounding and multi-line and multiphase grounding faults in 10 kV low-resistance grounding systems.This article first explains the principle of fault analysis for a 10 kV low-resistance grounding system.It emphasizes how to use the symmetrical component method to perform steady-state quantity analysis on various grounding faults in a low-resistance grounding system,and clarifies the distribution and relationship of key electrical quantities such as current and voltage during the fault.Subsequently,the direct and effective idea of solving the problem of high transient resistance ground fault is to reduce the setting value of the protection,and use a comparison of some characteristic between all outgoing lines to select the faulty line and the non-faulty line.At the same time,it also illustrates the effect of the line capacitance current on the protection setting value.Based on this idea,according to the ratio of the magnitude of the zero-sequence current between the 10 kV outgoing lines and the neutral point of the grounding transformer,a simple and efficient ratio rule,zero-sequence protection,is proposed,and the PSCAD simulation model is used to obtain the data required for the calculation example demonstrating.Furthermore,the impact of the zero-sequence overcurrent protection on the short-circuit transient process,abnormal device alarms,and zero-sequence transformer installation errors are analyzed,and the possible improvement of the ratio rule is made.Then by using Logistic regression algorithm to explain the principle of fault judgment,the classification of the data constructed by the ratio rule is verified and the feasibility of relay protection using the ratio rule is proved,besides being applied to the actual fault discrimination.By comparing the effects of logistic regression and the classification of the ratio rule,the advantages of using the ratio rule for fault analysis are explained.Finally,the idea of ratio rule is extended and applied to distance protection,which shows the flexibility and effectiveness of the ratio rule,and further demonstrates its ability as a simplified classifier.The zero-sequence protection proposed by the ratio rule in this paper is essentially an improved zero-sequence overcurrent protection,which retains the simple and practical advantages of zero-sequence overcurrent protection,and is easy to popularize and implement.The mathematical relationship between the current quantities is used to construct discriminants,so that each fault judgment of a 10 kV outgoing line is determined by the zero-sequence currents of all other 10 kV outgoing lines and the neutral point of the grounding transformer.Theoretically,the setting value of zero-sequence protection can be reduced to the line unbalance current,which can effectively protect the 10 kV low-resistance grounding system from high transition resistance and multi-line multi-phase grounding fault.